1. Field of the Invention
The present invention relates to a flow sensor used for measuring the flow rate of a gas, and more particularly to a flow sensor adapted for measuring the respiratory flow of a living body.
2. Description of Related Art
As instruments for measuring parameters related to the respiration of a patient while connected to an artificial respiration device, an anesthesia device, or the like, various types have heretofore been known. As an apparatus for measuring respiratory flow among these instruments, a differential pressure type flow measuring apparatus is widely used. The differential pressure type flow measuring apparatus measures flow rate based on differential pressure by arranging a resistance within a flow path and by detecting pressure before and past the resistance.
The construction of an exemplary conventional flow measuring apparatus of this type is shown in FIGS. 10 to 12. An apparatus shown in FIGS. 10 and 11 has been disclosed in U.S. Pat. No. 5,088,332, and is characterized as fixing a resistance within a flow path. In FIGS. 10 and 11, three vanes 2 are arranged on the inner circumference of a flow tube 1 at an angular interval of 120.degree. toward the center. Apertures 3 are formed on both surfaces of each vane 2, the surfaces being at the center portion toward which the three vanes 2 join and being on the outer sides of the vane as viewed in the axial direction. The apertures 3 on both sides are connected to directly communicate with tubes 4 that are formed within the vane 2 so as to be orthogonal to the axial direction. The tubes 4 are connected further to a measuring instrument 5.
In the aforementioned construction, pressures of a gas at before and past the vane 2, a fixed resistance, in the flow direction are sensed through the apertures 3 and the tubes 4, and a differential pressure is detected by the measuring instrument 5, whereby the flow rate of the gas flowing through the flow tube 1 can be measured. Since the vanes 2 are arranged radially at an equal angular interval around the apertures 3, the effects of an asymmetrical flow profile are automatically eliminated. In this example, channels 6 for satisfactorily guiding a stream of gas, which collides against the surfaces of the vanes 2, into the aperture 3 are arranged in the surfaces of the vanes 2.
An apparatus shown in FIG. 12 has been disclosed in U.S. Pat. No. 4,083,245. In this apparatus, a variable resistance is provided within the flow path. It may be noted that in FIG. 12, parts and components corresponding to the portions shown in FIG. 10 are denoted by the same reference numerals and that their descriptions will be omitted as appropriate. In FIG. 12, the flow tube 1 includes an entrance/exit portion 1a and an exit/entrance portion 1b. Both portions 1a, 1b are fixed to each other through flanges 11a, 11b. Pressure ports 12a, 12b are respectively arranged on both portions 1a, 1b in such a manner that the pressure ports 12a, 12b are opened onto the tube. These pressure ports 12a, 12b are connected to the measuring instrument 5 through the tubes 4, respectively.
An orifice membrane 13 serving as a variable resistance is held between the flanges 11a, 11b. The orifice membrane 13 is made of rubber or an elastic body, and has at the center thereof a notched flap 14, one end of the flap 14 being integrally connected to the orifice membrane 13.
In the aforementioned construction, the pressures at before and past the orifice membrane 13 as viewed in the flow direction are sensed through the pressure ports 12 and the tubes 4, and a differential pressure is detected by the measuring instrument 5, whereby the flow rate of a gas flowing through the flow tube 1 can be measured. At this time, the orifice opens only slightly for a relatively small flow of gas, thereby giving a relatively high resistance. When the flow rate of the gas has increased, the flap 14 is pushed outward as shown in FIG. 12, so that the aperture of the orifice is increased, so that resistance is reduced. As a result, resistance that increases with increasing flow rate can be suppressed to stay constant.
A flow sensor disposed in the flow measuring apparatus shown in FIGS. 10 and 11 has the vanes 2 serving as a fixed resistance arranged radially so as to be symmetrical with respect to the apertures 3 at the center. Therefore, the flow direction can be controlled independently of the gas flow profiles, which in turn permits highly reliable differential pressure sensing. However, since the vanes 2 are fixed, fluid resistance, which corresponds to a gauge pressure on the entrance side of the flow sensor, increases with increasing flow rate of the gas flowing in both directions. As a result, sensitivity to a differential pressure to be detected on the entrance/exit side of the flow sensor varies in accordance with the flow rates. That is, within a normal respiratory flow range of .+-.3 L/sec, the differential pressure sensitivity at low flow rates is low and the differential pressure sensitivity at high flow rates is high.
Further, since the flow tube 1 is connected to an endotracheal tube through an L connector, a Y piece, or the like, the amplitudes of pressure vibrations become large as shown in FIG. 15. As a result, noises appear in differential pressure outputs. This noise increases with increasing flow rate. To cope with this problem, correction based on electric processing must be made, which in turn makes it difficult to extract the true values of the differential pressure outputs. It may be noted that FIG. 14 shows a case where a laminar flow is caused to flow through a direct tube, exhibiting smaller noise than the case shown in FIG. 15.
A flow sensor disposed in the flow measuring apparatus shown in FIG. 12 has the resistance being designed as a variable resistance that is constructed of the orifice membrane 13 having the flap 14. Therefore, when the flow rate of a gas increases, resistance is decreased with the flap 14 having been opened, which in turn keeps the resistance of the orifice constant. However, as shown in FIG. 16, fluid resistance is decreased and so is differential pressure sensitivity at high flow rates. In this case, as shown in FIGS. 17 and 18, the effects of the connected tubes are small and noise is decreased compared with the case of the fixed resistance shown in FIG. 10. It may be noted that FIG. 17 shows a case where a laminar flow is caused to flow through a direct tube similarly to FIG. 14, and that FIG. 18 is a case where the flow tube is connected to an endotracheal tube similarly to FIG. 15.